We have determined the nucleotide sequence of the transforming gene of AMV region. Within the cellular substitution there is an open reading frame of 795 nucleotides starting with the initiation codon ATG and terminating with the triplet TAG. This would code for a protein of 265 amino acids with a molecular weight of approximately 30,000. The complete nucleotide sequence of the integrated proviral genome of avian myelocytomatosis virus (MC29) coding for gag-myc protein has been determined. By comparison of this nucleotide sequence with the helper virus as well as the c-myc region, it was possbile to localize junction points between helper viral and v-myc sequences. These studies demonstrate that the LTR sequence of MC29 is very similar to that of RSV, the viral genome has suffered extensive deletions in the gag, pol and env genes, the gag region can code for p19 and part of p27, the recombination between viral and cellular sequences occurred in the coding region of p27 such that the open reading frame extends for an additional stretch of 1278 bp resulting in a gag-myc hybrid protein, the open reading frame terminated within the v-myc region, 300 bases upstream of v-myc-helper viral junction, and the v-myc-helper viral junction at the 3' end occurred in the middle of env gene, rendering it defective.